This study assesses the physiological responses of copepods to variations in the quality of their diet and temperature and explores the variability of food quality in the field. Laboratory experiments were used to constrain the effects of food quality in terms of phytoplankton organic ratios N:P, C:N and C:P and fatty acid content, on Temora longicornis feeding, respiration, and egg production rates (EPR). Maximum metabolic rates, assimilation efficiency and gross growth efficiency coincided with an optimum diet of ~16N:1P defining it as the copepod threshold nutrient ratio. In the field, a response of zooplankton biomass to temporal variations in seston N:P ratio was found in the North Sea and in the North Pacific Subtropical Gyre. Field measurements revealed latitudinal and temporal changes in food quality at the chlorophyll maximum along an Atlantic Meridional Transect (AMT). Better quality food was found in the Temperate and Upwelling domains and included seston with lower C:N ratios, N:P ratios closer to Redfield values and a higher proportion of polyunsaturated fatty acids (PUFA), essential fatty acids such as 20:5(n-3) and 22:6(n-3), high (n-3):(n-6) ratios and lower proportion of saturated fatty acids (SAFA). Time-series data showed that food quantity and quality declined over time in the temperate North region of the Atlantic Ocean. Ship-board experiments were undertaken to determine the effects of temperature on respiration and EPR of some mesopelagic copepod species along an AMT. 75-85% of the variation in routine metabolic rates was explained by allometric and thermodynamic relationships. Within the range of temperatures measured, estimates of Ea (activation energy) and Q10 suggested high sensitivity to temperature variation. Our results highlight the importance of quantifying the effects of a variety of ecological parameters on the physiological responses of copepods to understand the processes that regulate zooplankton dynamics and their effects on biogeochemical cycles and trophic transfer of energy.